This invention relates to utilizing a gas permeable membrane to increase the concentration of a less preferentially permeable component of a mixture of gases which are selectively permeable through the membrane. More specifically, it relates to a method of increasing nitrogen concentration of air using an oxygen selectively permeable gas separation membrane.
Using a membrane to separate components of fluid mixtures is a well developed technology of presently great commercial significance. In general, membrane separation processes involve bringing a fluid feed mixture in contact with one side of a fluid permeable membrane. The composition of the membrane is chosen, inter alia, to provide that the components of interest in the fluid mixture permeate the membrane selectively. That is, they permeate the membrane at different rates. The preferentially permeable components permeate faster than the less preferentially permeable components. Consequently, the preferentially permeable components concentrate on the side of the membrane opposite the feed mixture in a mixture often referred to as the permeate composition. The composition on the feed mixture side of the membrane becomes deficient in the preferentially permeable components, and accordingly, concentrated in the less preferentially permeable components. This product mixture is frequently designated the retentate composition.
Transmembrane flux is largely influenced by a driving force defined by a difference in physical properties between the fluids on opposite sides of the membrane. For example, in membrane separation via permeation, the driving force is the difference in partial pressures of the fluid components in the feed and permeate compositions. The property difference can diminish to lower the driving force and reduce transmembrane flux under certain conditions, such as if the preferentially permeable component is not removed from the vicinity of the permeate side of the membrane. In that event, the concentration of the preferentially permeable component builds up and increases the partial pressure in the permeate to an amount that might approach the partial pressure in the feed. Flow of the component through the membrane will reduce and ultimately stop as the partial pressure difference drops to zero.
Often a retentate composition enriched in the less preferentially permeable components is the desired product of separation. In continuous membrane separations, the components in the permeate product usually attain steady state concentrations, assuming steady feed composition, flow rates and other operating conditions. This causes the retentate mixture to also have a steady state concentration which limits the maximum purity of the less preferentially permeable components that can be obtained in a single stage separation. The steady state can be shifted to higher purity for example by changing the stage cut, i.e., the ratio of permeate flux to feed flux. However, this usually reduces the overall flow through the membrane separation unit to unacceptably low productivity levels. Consequently, single stage membrane separation is understood in the art to be normally limited to achieving only slightly increased concentrations of less preferentially permeable components. The art has primarily relied on multiple stage membrane separations to obtain very high concentrations of gas components.
A sometimes favored technique aimed at increasing the high driving force across the membrane for boosting permeation rates calls for sweeping a fluid past the permeate side of the membrane. The sweep fluid thus carries the permeate fluid away from the region adjacent to the membrane which amplifies the driving force and promotes permeation.
U.S. Pat. No. 3,536,611 discloses a device and membrane separation method primarily for concentrating liquids, for example, increasing the octane number of gasoline stocks by selectively removing low-octane components from naphtha, and removing aromatics from kerosene. In the preferred embodiments the membrane is formed of capillary tubes which are arranged in a woven mat that encircles a central distributor tube within a shell. A sweep stream is introduced so as to diffuse radially through the interstitial spaces between the tubes. A fluid different from the feed, permeate and retentate is used for the sweep.
U.S. Pat. No. 3,499,062 discloses single and multi-stage membrane separation processes for purifying diverse fluids, such as increasing oxygen enriched air, separating methane from hydrocarbons, recovering hydrogen from mixtures with other gases and repurifying helium. The ""062 patent discloses use of a sweep flow which may be a portion of the inlet stream at lower pressure and moved through the permeate portion of a membrane separator in order to maintain desirable effective concentration gradients.
U.S. Pat. No. 5,226,932 discloses processes which include passing a feed gas through a membrane separator at superatmospheric pressure while passing a purge gas of either dry product or externally supplied dry gas through the permeate countercurrent to the feed gas.
U.S. Pat. No. 5,378,263 discloses multi-stage membrane systems for producing very high purity nitrogen from air, i.e., typically greater than 99%. It is taught that in some circumstances the efficiency of separation may be enhanced by using the permeate product from a third stage separation as a countercurrent purge stream for the permeate side of the first stage separation.
It would still be desirable to provide a simple and efficient membrane separation method for purifying less preferentially permeable components from gas mixtures to significantly higher concentrations than heretofore thought possible. Accordingly, there is now provided a method of increasing the concentration of a component of a gas mixture comprising
providing a selectively gas permeable membrane,
supplying a gaseous feed mixture comprising two components, one component being less preferentially permeable through the selectively gas permeable membrane than the other component,
contacting one side of the selectively gas permeable membrane with the gas mixture, thereby causing the two components to permeate the membrane to produce a retentate gas mixture in contact with the one side of the membrane having a first enriched concentration of the less preferentially permeable component relative to the gaseous feed mixture, and a permeate gas mixture in contact with the opposite side of the membrane, and
introducing into the permeate gas mixture a sweep flow of the gaseous feed mixture at a rate effective to produce a second enriched concentration of the less preferentially permeable component in the retentate gas mixture higher than the first enriched concentration
in which said concentration higher than the enriched concentration is produced in a single stage membrane separation.
There is also provided according to this invention a single stage membrane separation apparatus for producing an enriched gas mixture comprising
a selectively gas permeable membrane,
means for supplying a gaseous feed mixture comprising two components, one component being less preferentially permeable through the selectively gas permeable membrane than the other component,
means for contacting one side of the selectively gas permeable membrane with the gaseous feed mixture, and thereby causing the two components to permeate the membrane to produce a retentate gas mixture in contact with the one side of the membrane having a first enriched concentration of the less preferentially permeable component relative to the gaseous feed mixture and a permeate gas mixture in contact with the opposite side of the membrane, and
means for introducing into the permeate gas mixture a sweep flow of the gaseous feed mixture at a rate effective to produce a second enriched concentration of the less preferentially permeable component in the retentate gas mixture higher than the first enriched concentration.